Hot gas defrost system using hot gas from low temperature compressor

ABSTRACT

A refrigeration system includes at least one low-temperature evaporator, at least one medium-temperature evaporator, one or more low-temperature compressors, and one or more medium-temperature compressors. A controllable valve positioned downstream from the one or more low-temperature compressors directs flow of refrigerant from the low-temperature compressor(s) to one or both of (i) the medium-temperature compressor(s) and (ii) one or more evaporators based on an operation mode of evaporators. A controller is communicatively coupled to the controllable valve. A controller determines that operation of a first evaporator in a defrost mode is indicated and causes the first evaporator to operate in the defrost mode by adjusting the controllable valve to direct a portion of the received compressed refrigerant to the first evaporator.

TECHNICAL FIELD

This disclosure relates generally to refrigeration systems. Moreparticularly, in certain embodiments, this disclosure relates to hot gasdefrost system using hot gas from low temperature compressor.

BACKGROUND

Refrigeration systems are used to regulate environmental conditionswithin an enclosed space. Refrigeration systems are used for a varietyof applications, such as in supermarkets and warehouses, to cool storeditems. For example, refrigeration systems may provide cooling operationsfor refrigerators and freezers.

SUMMARY OF THE DISCLOSURE

During operation of refrigeration systems, ice may build up onevaporators. These evaporators need to be defrosted to remove icebuildup and prevent loss of performance. Previous evaporator defrostprocesses are limited in terms of their efficiency and effectiveness.For example, using previous technology, defrost processes may take arelatively long time and consume a relatively large amount of energy. Insome cases, previous technology may be incapable of providing adequatedefrosting, for instance, in cases where a relatively large number ofevaporators need to be defrosted in a multiple-evaporator refrigerationsystem.

This disclosure provides technical solutions to the problems of previoustechnology, including those described above. For example, arefrigeration system is described that facilitates improved evaporatordefrost using discharge gas from one or more low-temperature (LT)compressors located downstream of a low temperature portion of therefrigeration system. While one or a portion of the evaporators of therefrigeration system are operating in a normal refrigeration mode, otherevaporator(s) can be operated in a defrost mode using hot gas producedby the refrigeration process. A check valve is positioned in refrigerantconduit connecting an outlet of the LT compressor(s) to a flash tank andis configured to open if a pressure of refrigerant from the LTcompressor(s) exceeds a threshold value. Embodiments of this disclosuremay provide improved defrost operations to evaporators of refrigerationsystems, such as CO₂ refrigeration systems. In certain embodiments, therefrigeration system does not require specialized high pressureevaporator components because hot gas is provided at a moderate pressurefrom the LT compressor(s). In certain embodiments, system complexity andcost is decreased, for example, because a pressure-regulating valve isnot used at the discharge of the LT compressor(s). Certain embodimentsmay include none, some, or all of the above technical advantages. One ormore other technical advantages may be readily apparent to one skilledin the art from the figures, descriptions, and claims included herein.

In an embodiment, a refrigeration system includes a plurality ofevaporators that include at least one low-temperature evaporator and atleast one medium-temperature evaporator. The refrigeration systemincludes one or more low-temperature compressors configured to compressrefrigerant received from the at least one low-temperature evaporator.The refrigeration system includes one or more medium-temperaturecompressors configured to compress refrigerant received from the atleast one medium-temperature evaporator. The refrigeration systemincludes a controllable valve positioned downstream from the one or morelow-temperature compressors. The controllable valve is configured toreceive the compressed refrigerant from the one or more low-temperaturecompressors and direct flow of the received refrigerant to one or bothof (i) the one or more medium-temperature compressors positioneddownstream from the controllable valve and (ii) one or more evaporatorsof the plurality of evaporators based on an operation mode of theplurality of evaporators. A controller is communicatively coupled to thecontrollable valve. The controller determines that operation of a firstevaporator of the plurality of evaporators in a defrost mode isindicated and, after determining that operation of the first evaporatorin the defrost mode is indicated, causes the first evaporator to operatein the defrost mode by adjusting the controllable valve to direct aportion of the received compressed refrigerant to the first evaporator.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, referenceis now made to the following description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagram of an example refrigeration system of thisdisclosure configured to operate evaporators in a refrigeration mode;

FIG. 2 is a diagram of the example refrigeration system of FIG. 1configured to operate an evaporator in a defrost mode; and

FIG. 3 is a flowchart of an example method of operating therefrigeration system of FIGS. 1 and 2 to provide improved evaporatordefrost.

DETAILED DESCRIPTION

Embodiments of the present disclosure and its advantages are bestunderstood by referring to FIGS. 1-3 of the drawings, like numeralsbeing used for like and corresponding parts of the various drawings.

As described above, prior to this disclosure, defrost operations ofrefrigeration systems suffered from certain inefficiencies anddrawbacks. The refrigeration system of this disclosure providesimprovements in defrost performance and energy efficiency. Therefrigeration system of this disclosure may be a CO₂ refrigerationsystem. CO₂ refrigeration systems may differ from conventionalrefrigeration systems in that these systems circulate refrigerant thatmay become a supercritical fluid (i.e., where distinct liquid and gasphases are not present) above the critical point. As an example, thecritical point for carbon dioxide (CO₂) is 31° C. and 73.8 MPa, andabove this point, CO₂ becomes a homogenous mixture of vapor and liquidthat is called a supercritical fluid. This unique characteristic oftranscritical refrigerants is associated with certain operationaldifferences between transcritical and conventional refrigerationsystems. For example, transcritical refrigerants are typicallyassociated with discharge temperatures that are higher than theircritical temperatures and discharge pressures that are higher than theircritical pressures. When a transcritical refrigerant is at or above itscritical temperature and/or pressure, the refrigerant may become a“supercritical fluid”—a homogenous mixture of gas and liquid.Supercritical fluid does not undergo phase change process (vapor toliquid) in a gas cooler as occurs in a condenser of a conventionalrefrigeration system circulating traditional refrigerant. Rather,supercritical fluid cools down to a lower temperature in the gas cooler.Stated differently, the gas cooler in a CO₂ transcritical refrigerationsystem may receive and cool supercritical fluid, and the transcriticalrefrigerant undergoes a partial state change from gas to liquid as it isdischarged from an expansion valve.

Refrigeration System

FIGS. 1 and 2 illustrate an example refrigeration system 100 configuredfor improved defrost operation. The refrigeration system 100 shown inFIG. 1 is configured to operate evaporators 116, 128 in a refrigerationmode, such that the evaporators 116, 128 provide cooling to acorresponding space, such as a freezer and deep freeze, respectively(not shown for clarity and conciseness). FIG. 2 illustrates the examplerefrigeration system 100 when configured for operation of evaporator 128in a defrost mode, such that evaporator 128 is defrosted and evaporator116 still provides cooling to a space. When at least one of theevaporators 116, 128 is operated in the defrost mode, refrigerant fromone or more low-temperature (LT) compressors 138 is provided to theevaporators 116, 128 to facilitate defrosting of the evaporators 116,128. The refrigerant removes ice buildup from coils of the evaporator(s)116, 128.

Refrigeration system 100 includes one or more medium-temperature (MT)compressors 102, refrigerant conduit subsystem 104, controllable valve106, check valve 118, gas cooler 108, flash tank 112, one or more MTevaporators 116 and corresponding valves 114, 120, 122, 124, one or moreLT evaporators 128 and corresponding valves 126, 132, 134, 136, one ormore LT compressors 138, a valve 140, a flash-gas bypass valve 142, andcontroller 150. In some embodiments, refrigeration system 100 is atranscritical refrigeration system that circulates a transcriticalrefrigerant such as CO₂.

The MT compressor(s) 102 are configured to compress refrigerantdischarged from the MT evaporator(s) 116 that are operating inrefrigeration mode (as shown in FIGS. 1 and 2 ) and provide supplementalcompression to refrigerant discharged from any of the LT evaporators 128that are operating in refrigeration mode (as shown in FIG. 1 ).Refrigeration system 100 may include any suitable number of MTcompressors 102. MT compressor(s) 102 may vary by design and/or bycapacity. For example, some compressor designs may be more energyefficient than other compressor designs, and some MT compressors 102 mayhave modular capacity (e.g., a capability to vary capacity). Thecontroller 150 is in communication with the MT compressors 102 andcontrols their operation.

Refrigerant conduit subsystem 104 facilitates the movement ofrefrigerant (e.g., CO₂) through a refrigeration cycle such that therefrigerant flows in the refrigeration mode as illustrated by the arrowsin FIG. 1 . The refrigerant conduit subsystem 104 includes conduit,tubing, and the like that facilitates the movement of refrigerantbetween components of the refrigeration system 100.

Valve 106 is generally a motorized or otherwise electronicallycontrollable valve, such as a motorized ball valve, solenoid valve, orthe like. Valve 106 receives compressed refrigerant from the LTcompressor(s) 128 and is adjustable to control flow of refrigeranttowards one or more of the MT and/or LT evaporators 116, 128 to providedefrost. The controller 150 is in communication with valve 106 andcontrols its operation.

Gas cooler 108 is generally operable to receive refrigerant (e.g., fromMT compressor(s) 102) and apply a cooling stage to the receivedrefrigerant. In some embodiments, gas cooler 108 is a heat exchangercomprising cooler tubes configured to circulate the received refrigerantand coils through which ambient air is forced. Inside gas cooler 108,the coils may absorb heat from the refrigerant, thereby cooling therefrigerant.

Flash tank 112 is configured to receive mixed-state refrigerant andseparate the received refrigerant into flash gas and liquid refrigerant.Flash tank 112 may include one or more tanks operable to holdrefrigerant at least temporarily. Typically, the flash gas collects nearthe top of flash tank 112, and the liquid refrigerant is collected inthe bottom of flash tank 112. A valve 110 may be disposed at or near aninlet of the flash tank 112 to reduce pressure of refrigerant receivedby the flash tank 112. When both evaporators 116 and 128 are operated inrefrigeration mode (see FIG. 1 ), the liquid refrigerant flows fromflash tank 112 and provides cooling to the MT evaporator 116 and LTevaporator 128. When evaporator 128 is operated in defrost mode (seeFIG. 2 ), hot gas refrigerant provided to defrost evaporator 128 isprovided to flash tank 112. Valve 140 may adjust the pressure ofrefrigerant provided to the flash tank 112 as appropriate to facilitaterefrigerant flow as illustrated in FIG. 2 .

When operated in refrigeration mode (see FIG. 1 ), the MT evaporator 116receives cooled liquid refrigerant from the flash tank 112 and uses thecooled refrigerant to provide cooling. As an example, the evaporator 116may be part of a refrigerated case and/or cooler for storing items thatmust be kept at particular temperatures. The refrigeration system 100may include any appropriate number of MT evaporators 116 with the sameor a similar configuration to that shown for the example MT evaporator116 shown in FIGS. 1 and 2 .

Each of the one or more MT evaporators 116 has corresponding valves 114,120, 122, 124 to facilitate operation of the MT evaporator 116 in arefrigeration mode and a defrost mode. Valve 114 may be an expansionvalve. Expansion valve 114 may be configured to receive liquidrefrigerant from flash tank 112 and reduce the pressure of the receivedrefrigerant. In some embodiments, this reduction in pressure causes someof the refrigerant to vaporize. Valves 120, 122, 124 may be anyappropriate motorized or electronically controllable valves, such asmotorized ball valves, solenoid valves, and/or the like. The controller150 is in communication with valves 114, 120, 122, 124 and controlstheir operation.

When the MT evaporator 116 is operated in the refrigeration modeillustrated in FIGS. 1 and 2 , the first valve 114 upstream of theevaporator 116 is open and the second valve 120 downstream of theevaporator 116 is open. The third valve 124 and fourth valve 122 areboth closed. In this configuration, the liquid refrigerant from flashtank 112 flows through expansion valve 114, where the pressure of therefrigerant is decreased, before it reaches the evaporator 116.Expansion valve 114 may be configured to achieve a refrigeranttemperature into the evaporator 116 at a predefined temperature for agiven application (e.g., about −6° C.). Refrigerant from the MTevaporator 116 that is operating in refrigeration mode is provided tothe one or more MT compressors 102.

When the MT evaporator 116 is operated in the defrost mode (not shownfor conciseness), valve 106 is adjusted such that at least a portion ofcompressed refrigerant from the LT compressor(s) 138 is directed towardsthe MT evaporator 116. The first valve 114 upstream of the evaporator116 is closed, and the second valve 120 downstream of the evaporator 116is closed. Third valve 124 and fourth valve 122 are opened to allow flowof compressed refrigerant from the valve 106 toward the MT evaporator116. In this configuration, heated refrigerant from LT compressor(s) 138flows through the evaporator 116 and defrosts the evaporator 116.Refrigerant exiting the evaporator 116 flows through the opened valve124 and to optional expansion valve 140. Expansion valve 140, ifpresent, expands the refrigerant (i.e., decreases pressure of therefrigerant) before it flows back into the flash tank 112. Expansionvalve 140 may be the same as or similar to expansion valve 114,described above.

Once defrost mode operation is complete, the controller 150 may enddefrost mode operation and return to refrigeration mode operation byclosing valves 122 and 124 and opening valves 114 and 120. In someembodiments, the controller 150 may cause defrost mode to end after apredefined period of time included in the instructions 158 and/orschedule 162. In some embodiments, the controller 150 may cause defrostmode operation to end after predefined conditions indicated in theinstructions 158 are reached (e.g., after a temperature and/or pressure160 measured by sensor 144 reaches a threshold 164).

The LT evaporator 128 is generally similar to the MT evaporator 116 butis configured to operate at lower temperatures (e.g., for deep freezingapplications near about −30° C. or the like). When operated inrefrigeration mode (see FIG. 1 ), the LT evaporator 128 receives cooledliquid refrigerant from the flash tank 112 and uses the cooledrefrigerant to provide cooling. As an example, the evaporator 128 may bepart of a deep freezer for relatively long-term storage of perishableitems that must be kept at particular temperatures. For clarity andconciseness, the components of a single LT evaporator 128 areillustrated. The refrigeration system 100 may include any appropriatenumber of LT evaporators 128 with corresponding valves 126, 132, 134,136.

The LT evaporator 128 includes valves 126, 132, 134, 136 to facilitateoperation of the LT evaporator 128 in a refrigeration mode (see FIG. 1 )and a defrost mode (see FIG. 2 ). Valve 126 may be an expansion valvethat is the same as or similar to valve 114, described above. Expansionvalve 126 may be configured to receive liquid refrigerant from flashtank 112 and reduce the pressure of the received refrigerant. In someembodiments, this reduction in pressure causes some of the refrigerantto vaporize. Valves 132, 134, 136 may be any appropriate motorized orelectronically controllable valves, such as motorized ball valves,solenoid valves, and/or the like (e.g., the same as or similar to valve120, 122, 124, described above). The controller 150 is in communicationwith valves 126, 132, 134, 136 and controls their operation.

When the LT evaporator 128 is operated in the refrigeration modeillustrated in FIG. 1 , the first valve 126 upstream of the evaporator128 is open and the second valve 132 downstream of the evaporator 128 isopen. The third valve 136 and fourth valve 134 are both closed. In thisconfiguration, the liquid refrigerant from flash tank 112 flows throughexpansion valve 126, where the pressure of the refrigerant is decreased,before it reaches the evaporator 128. Expansion valve 126 may beconfigured to achieve a refrigerant temperature into the evaporator 128at a predefined temperature for a given application (e.g., about −30°C.). Refrigerant from the LT evaporator 128 that is operating inrefrigeration mode is provided to the one or more LT compressors 138.

When the LT evaporator 128 is operated in the defrost mode of FIG. 2 ,the valve 106 is adjusted such that at least a portion of compressedrefrigerant from the LT compressor(s) 138 is directed towards the LTevaporator 128. The first valve 126 upstream of the evaporator 128 isclosed, and the second valve 132 downstream of the evaporator 128 isclosed. Third valve 136 and fourth valve 134 are opened to allow flow ofcompressed refrigerant from the valve 106 toward the LT evaporator 128.In this configuration, heated refrigerant from LT compressor(s) 138flows through the evaporator 128 and defrosts the evaporator 128.Refrigerant exiting the evaporator 128 flows through the opened valve136 and to optional expansion valve 140, described above.

Once defrost mode operation is complete, the controller 150 may enddefrost mode operation and return to refrigeration mode operation byclosing valves 134 and 136 and opening valves 126 and 132, as shown inthe example of FIG. 1 . In some embodiments, the controller 150 maycause defrost mode to end after a predefined period of time included inthe instructions 158 and/or schedule 162. In some embodiments, thecontroller 150 may cause defrost mode operation to end after predefinedconditions indicated in the instructions 158 are reached (e.g., after atemperature and/or pressure 160 measured by sensor 146 reaches athreshold 164).

The temperature and/or pressure sensors 144, 146 may be disposed on, in,or near the corresponding evaporators 116, 128 or refrigerant conduitconnected to the evaporators 116, 128. Information from sensors 144, 146may assist in determining when operation in defrost mode is appropriateor should be ended. For example, if the temperature and/or pressure 160measured by sensors 144, 146 indicates potential freezing of the MTevaporator 116 and/or LT evaporator 128, defrost mode operation may beindicated. In some cases, defrost mode operation is determined to beindicated based on a schedule 162 (e.g., defrost mode operation may beperformed at certain predefined time intervals or at certain times).

Valves 114, 120, 122, and 124 for the MT evaporator 116 and valves 126,132, 134, and 136 for the LT evaporator 128 may be in communication withcontroller 150, and the controller 150 may provide instructions foradjusting these valves 114, 120, 122, 124, 126, 132, 134, 136 to open orclosed positions to achieve the configurations described above forrefrigeration mode operation and defrost mode operation. For example,instructions 158 implemented by the processor 152 of the controller 150may determine that operation of the MT evaporator 116 and/or the LTevaporator 128 in a defrost mode is indicated. For example, instructions158 stored by the controller 150 may indicate that defrost modeoperation is needed on a certain schedule 162 or at a certain time. Asanother example, a temperature and/or pressure 160 of the evaporators116, 128 may indicate that defrost mode operation is needed (e.g.,because the temperature and/or pressure 160 indicates that expectedcooling performance or efficiency is not being obtained).

A check valve 118 is positioned in refrigerant conduit of the conduitsubsystem 104 coupling an outlet of the LT compressor(s) 138 to an inletof the flash tank 112. The check valve 118 is configured to allow flowof refrigerant from the LT compressor(s) 138 to the flash tank 112 whena pressure difference across the check valve exceeds a threshold value(e.g., of about 6 bar to 10 bar). In other words, the check valve 118 isa one-way valve and prevents flow from the flash tank 112 to the conduitsubsystem 104 connecting to the LT compressor(s) 138 and only allowsflow to the flash tank 112 if a threshold pressure is reached. If thepressure difference across the check valve 118 is below the thresholdvalue, flow is prevented to the flash tank 112.

Flash gas bypass valve 142 may be located in refrigerant conduit of theconduit subsystem 104 connecting the flash tank 112 to the MTcompressor(s) 102 and configured to open and close to permit or restrictthe flow of flash gas discharged from flash tank 112. In someembodiments, controller 150 controls the opening and closing of flashgas bypass valve 142. As depicted in FIGS. 1 and 2 , closing flash gasbypass valve 142 may restrict flash gas from flowing to MT compressor(s)102, and opening flash gas bypass valve 142 may permit flow of flash gasto MT compressor(s) 102.

As described above, controller 150 is in communication with at leastvalve 106; valves 114, 120, 122, and 124 of the MT evaporator 116;valves 126, 132, 134, and 136 of the LT evaporator 128; and compressors102, 138. The controller 150 adjusts operation of components of therefrigeration system 100 to operate the evaporators 116, 128 inrefrigeration mode or defrost mode, as described herein. The controller150 includes a processor 152, memory 154, and input/output (I/O)interface 156. The processor 152 includes one or more processorsoperably coupled to the memory 154. The processor 152 is any electroniccircuitry including, but not limited to, state machines, one or morecentral processing unit (CPU) chips, logic units, cores (e.g., amulti-core processor), field-programmable gate array (FPGAs),application specific integrated circuits (ASICs), or digital signalprocessors (DSPs) that communicatively couples to memory 154 andcontrols the operation of refrigeration system 100.

The processor 152 may be a programmable logic device, a microcontroller,a microprocessor, or any suitable combination of the preceding. Theprocessor 152 is communicatively coupled to and in signal communicationwith the memory 154. The one or more processors are configured toprocess data and may be implemented in hardware or software. Forexample, the processor 152 may be 8-bit, 16-bit, 32-bit, 64-bit or ofany other suitable architecture. The processor 152 may include anarithmetic logic unit (ALU) for performing arithmetic and logicoperations, processor registers that supply operands to the ALU andstore the results of ALU operations, and a control unit that fetchesinstructions from memory 154 and executes them by directing thecoordinated operations of the ALU, registers, and other components. Theprocessor 152 may include other hardware and software that operates toprocess information, control the refrigeration system 100, and performany of the functions described herein (e.g., with respect to FIGS. 1-3). The processor 152 is not limited to a single processing device andmay encompass multiple processing devices. Similarly, the controller 150is not limited to a single controller but may encompass multiplecontrollers.

The memory 154 includes one or more disks, tape drives, or solid-statedrives, and may be used as an over-flow data storage device, to storeprograms when such programs are selected for execution, and to storeinstructions 158 and data that are read during program execution. Thememory 154 may be volatile or non-volatile and may include ROM, RAM,ternary content-addressable memory (TCAM), dynamic random-access memory(DRAM), and static random-access memory (SRAM). The memory 154 isoperable (or configured) to store information used by the controller 150and/or any other logic and/or instructions for performing the functiondescribed in this disclosure.

The I/O interface 156 is configured to communicate data and signals withother devices. For example, the I/O interface 156 may be configured tocommunicate electrical signals with components of the refrigerationsystem 100 including valves 106, 114, 120, 122, 124, 126, 132, 134, 136,140, 142; sensors 144, 146; and compressors 102, 138. The I/O interface156 may be configured to communicate with other devices and systems. TheI/O interface 156 may provide and/or receive, for example, compressorspeed signals, compressor on/off signals, valve open/close signals,temperature signals, pressure signals, temperature setpoints,environmental conditions, and an operating mode status for therefrigeration system 100 and send electrical signals to the componentsof the refrigeration system 100. The I/O interface 156 may include portsor terminals for establishing signal communications between thecontroller 150 and other devices. The I/O interface 156 may beconfigured to enable wired and/or wireless communications.

Although this disclosure describes and depicts refrigeration system 100including certain components, this disclosure recognizes thatrefrigeration system 100 may include any suitable components. As anexample, refrigeration system 100 may include one or more additionalsensors configured to detect temperature and/or pressure information.

In an example operation of the refrigeration system 100, therefrigeration system 100 is initially operating with both evaporators116, 128 in the refrigeration mode, as illustrated in FIG. 1 . In thismode, first valve 126 and second valve 132 of LT evaporator 128 areopen, and third valve 136 and fourth valve 134 are closed. At some pointduring operation of the refrigeration system 100, the controller 150determines that defrost mode operation is needed for the LT evaporator128. For example, the LT evaporator 128 may be scheduled for defrost atthe time that has just been reached. After determining that the defrostmode operation is indicated, the controller 150 causes the LT evaporator128 to be configured according to FIG. 2 . In other words, thecontroller 150 causes the valve 106 to allow a portion of refrigerantfrom the LT compressor(s) 138 to flow towards the LT evaporator 128(e.g., by partially closing valve 106). First valve 126 and second valve132 are closed, and third valve 136 and fourth valve 134 are opened.

Once defrost of the LT evaporator 128 is complete (e.g., because defrostmode operation has been performed for a predefined period of time and/ora threshold pressure and/or temperature 160 of the LT evaporator 128 hasbeen reached), the controller 150 causes the LT evaporator 128 tooperate in the refrigeration mode, as illustrated in FIG. 1 anddescribed above.

Example Method of Operation

FIG. 3 illustrates an example method 300 of operating the refrigerationsystem 100 described above with respect to FIGS. 1 and 2 . The method300 may be implemented using the processor 152, memory 154, and I/Ointerface 156 of the controller 150 of FIGS. 1 and 2 . The method 300may begin at operation 302 where the controller 150 initially operatesthe evaporator 116, 128 in the refrigeration mode. At operation 304, thecontroller 150 determines whether defrost mode is indicated for any ofthe evaporators 116, 128. For example, the controller 150 may determinewhether the instructions 158 and/or schedule 162 indicate that a defrostcycle is needed for one of the evaporators 116, 128. As another example,the controller 150 may determine whether a temperature and/or pressure160 measured at an evaporator 116, 128 indicates decreased performance(e.g., if a target or threshold value 164 of temperature and/or pressure160 is not being reached). This behavior may indicate that a defrostmode operation is indicated. If defrost mode is not indicated, thecontroller 150 returns to operation 302 and continues to operate theevaporators 116, 128 in the refrigeration mode. If defrost modeoperation is indicated, the controller 150 proceeds to operation 306.

At operation 306, the controller 150 causes the evaporator 116, 128determined at operation 304 to be operated in the defrost mode. Forinstance, if defrost of the LT evaporator 128 is needed, the controller150 may cause the valve 106 to allow a portion of refrigerant from theLT compressor(s) 138 to flow towards the LT evaporator 128. First valve126 and second valve 132 are closed, and third valve 136 and fourthvalve 134 are opened. This achieves the defrost mode configuration ofevaporator 128 illustrated in FIG. 2 .

At operation 308, the controller 150 determines whether defrost modeoperation of the evaporator 128 is complete. For example, the controller150 may determine whether defrost mode operation has been performed fora predefined period of time indicated by schedule 162 and/or if athreshold value 164 is reached for a pressure and/or temperature 160 ofthe LT evaporator 128. If defrost mode operation is not complete, thecontroller continues to operate in the defrost mode at operation 306.Once defrost mode operation is complete, the controller 150 returns tooperation 302 and operates the evaporator 128 in the refrigeration mode.

Modifications, additions, or omissions may be made to method 300depicted in FIG. 3 . Method 300 may include more, fewer, or otheroperations. For example, operations may be performed in parallel or inany suitable order. While at times discussed as controller 150,refrigeration system 100, or components thereof performing theoperations, any suitable refrigeration system or components of therefrigeration system may perform one or more operations of the method300.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants notethat they do not intend any of the appended claims to invoke 35 U.S.C. §112(f) as it exists on the date of filing hereof unless the words “meansfor” or “step for” are explicitly used in the particular claim.

What is claimed is:
 1. A refrigeration system, comprising: a plurality of evaporators comprising at least one low-temperature evaporator and at least one medium-temperature evaporator; one or more low-temperature compressors configured to compress refrigerant received from the at least one low-temperature evaporator; one or more medium-temperature compressors configured to compress refrigerant received from the at least one medium-temperature evaporator; a controllable valve positioned downstream from the one or more low-temperature compressors, the controllable valve configured to receive the compressed refrigerant from the one or more low-temperature compressors and direct flow of the received refrigerant to one or both of (i) the one or more medium-temperature compressors positioned downstream from the controllable valve and (ii) one or more evaporators of the plurality of evaporators based on an operation mode of the plurality of evaporators; and a controller communicatively coupled to the controllable valve, wherein the controller is configured to: determine that operation of a first evaporator of the plurality of evaporators in a defrost mode is indicated; and after determining that operation of the first evaporator in the defrost mode is indicated, cause the first evaporator to operate in the defrost mode by adjusting the controllable valve to direct a portion of the received compressed refrigerant to the first evaporator.
 2. The refrigeration system of claim 1, wherein the controller is further configured to cause the first evaporator to operate in the defrost mode by at least partially closing the controllable valve.
 3. The refrigeration system of claim 1, further comprising: a gas cooler configured to receive refrigerant from the one or more medium-temperature compressors and facilitate heat transfer from the received refrigerant; and a flash tank located downstream from the gas cooler and configured to receive the refrigerant and store at least a portion of the received refrigerant.
 4. The refrigeration system of claim 3, further comprising: a first valve located upstream from the first evaporator in refrigerant conduit coupling a liquid outlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the first valve is open; and a second valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant towards the one or more medium-temperature compressors, wherein, when the first evaporator is operating in the refrigeration mode, the second valve is open; wherein the controller is further configured to cause the first evaporator to operate in the defrost mode by causing the first valve to close and causing the second valve to close.
 5. The refrigeration system of claim 4, further comprising: a third valve located upstream from the first evaporator in refrigerant conduit coupling an inlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the third valve is closed; and a fourth valve located downstream from the first evaporator in refrigerant conduit coupling the first evaporator to the controllable valve, wherein, when the first evaporator is operating in the refrigeration mode, the fourth valve is closed; wherein the controller is further configured to cause the first evaporator to operate in the defrost mode by causing the third valve to open and causing the fourth valve to open.
 6. The refrigeration system of claim 3, further comprising a check valve positioned in refrigerant conduit coupling an outlet of the one or more low-temperature compressors to an inlet of the flash tank, wherein the check valve is configured to allow flow of refrigerant from the one or more low-temperature compressors to the flash tank when a pressure difference across the check valve exceeds a threshold value.
 7. The refrigeration system of claim 1, wherein the controller is further configured to: determine that defrost mode operation of the first evaporator is complete; and after determining that defrost mode operation of the first evaporator is complete, cause the first evaporator to operate in a refrigeration mode.
 8. The refrigeration system of claim 1, wherein, while the first evaporator is caused to operate in the defrost mode, a second evaporator of the plurality of evaporators is caused to operate in a refrigeration mode.
 9. A method of operating a refrigeration system, the method comprising: operating a first evaporator of a plurality of evaporators in a refrigeration mode; determining that operation of the first evaporator in a defrost mode is indicated; and after determining that operation of the first evaporator in the defrost mode is indicated, causing the first evaporator to operate in the defrost mode by: adjusting a controllable valve positioned in refrigerant conduit coupling one or more low-temperature compressors of the refrigeration system to one or more medium-temperature compressors of the refrigeration system; and directing at least a portion of refrigerant compressed by the one or more low-temperature compressors to the first evaporator.
 10. The method of claim 9, further comprising causing the first evaporator to operate in the defrost mode by at least partially closing the controllable valve.
 11. The method claim 9, further comprising: receiving, by a gas cooler of the refrigeration system, refrigerant from the one or more compressors and cooling the refrigerant; and receiving and storing, by a flash tank, the cooled refrigerant.
 12. The method of claim 11, further comprising causing the first evaporator to operate in the defrost mode by: closing a first valve located upstream from the first evaporator in refrigerant conduit coupling a liquid outlet of the flash tank to the first evaporator; and closing a second valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant towards the one or more medium-temperature compressors.
 13. The method of claim 12, further comprising: opening a third valve located upstream from the first evaporator in refrigerant conduit coupling an inlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the third valve is closed; and opening a fourth valve located downstream from the first evaporator in refrigerant conduit coupling the first evaporator to the controllable valve, wherein, when the first evaporator is operating in the refrigeration mode, the fourth valve is closed.
 14. The method of claim 11, further comprising, by a check valve positioned in refrigerant conduit coupling an outlet of the one or more low-temperature compressors to an inlet of the flash tank, allowing flow of refrigerant from the one or more low-temperature compressors to the flash tank when a pressure difference across the check valve exceeds a threshold value.
 15. The method of claim 9, further comprising: determining that defrost mode operation of the first evaporator is complete; and after determining that defrost mode operation of the first evaporator is complete, causing the first evaporator to operate in the refrigeration mode.
 16. The method of claim 9, wherein, while causing the first evaporator to operate in the defrost mode, causing a second evaporator of the plurality of evaporators to operate in a refrigeration mode.
 17. A controller of a refrigeration system, the controller comprising: an input/output interface communicatively coupled to a controllable valve positioned downstream from one or more low-temperature compressors of the refrigeration system, wherein the controllable valve is configured to receive the compressed refrigerant from the one or more low-temperature compressors and direct flow of the received refrigerant to one or both of (i) one or more medium-temperature compressors positioned downstream from the controllable valve and (ii) one or more evaporators of the plurality of evaporators based on an operation mode of the plurality of evaporators; and a processor configured to: determine that operation of a first evaporator of a plurality of evaporators of the refrigeration system in a defrost mode is indicated; and after determining that operation of the first evaporator in the defrost mode is indicated, cause the first evaporator to operate in the defrost mode by adjusting the controllable valve to direct a portion of the received compressed refrigerant to the first evaporator.
 18. The controller of claim 17, wherein the processor is further configured to cause the first evaporator to operate in the defrost mode by at least partially closing the controllable valve.
 19. The controller of claim 17, wherein: the input/output interface communicatively coupled to: a first valve located upstream from the first evaporator in refrigerant conduit coupling a liquid outlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the first valve is open; and a second valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant towards the one or more medium-temperature compressors, wherein, when the first evaporator is operating in the refrigeration mode, the second valve is open; and the processor is further configured to cause the first evaporator to operate in the defrost mode by causing the first valve to close and causing the second valve to close.
 20. The controller of claim 19, wherein: the input/output interface is further communicatively coupled to: a third valve located upstream from the first evaporator in refrigerant conduit coupling an inlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the third valve is closed; and a fourth valve located downstream from the first evaporator in refrigerant conduit coupling the first evaporator to the controllable valve, wherein, when the first evaporator is operating in the refrigeration mode, the fourth valve is closed; and the processor is further configured to cause the first evaporator to operate in the defrost mode by causing the third valve to open and causing the fourth valve to open. 